After spending a few sessions using tesserae to make mosaics, we've gone a bit more industrial now, and are using pebbles and cement!
The idea is to make stepping stones, suitable for putting out into a garden.
We first make a base of clay in a plastic container, then push pebbles into the clay to create a design.
This is a bit like making an upside-down cake, and you have to remember which side you will be looking at (the areas of the pebbles that are in the clay).
We then make up some concrete, using cement, small ballast, and coarse sand. This is poured onto the 'back' of the pebble design, and allowed to go off.
Finally, we have to tip the whole thing out of the container, and get rid of the clay. It's a great technique, and we had very good results from it.